The present invention relates generally to the field of optoelectronics. In particular, the present invention relates to dry-films which are suitable for use in forming optical components such as waveguides. As well, the invention relates to methods of forming optical components with a dry-film. The invention additionally relates to methods of forming electronic devices such as printed wiring boards having optical functionality.
Signal transmission using pulse sequences of light is becoming increasingly important in high-speed communications and data transfer. For example, optical integrated circuits are gaining importance for high bandwidth optical interconnects. As a result, the integration of optical components such as waveguides, filters, optical interconnects, lenses, diffraction gratings, and the like, is becoming increasingly important.
The incorporation of optical layers in the form of embedded optical waveguides into printed wiring boards is known. For example, U.S. Pat. Nos. 6,731,857 and 6,842,577 to Shelnut et al, disclose embedded optical waveguides formed using silsesquioxane chemistry on a printed wiring board substrate. The optical waveguides include a core and a clad surrounding the core, with optical radiation propagating in the core due to its higher index of refraction as compared to the clad.
Embedded optical waveguides are typically formed by coating a bottom clad layer over a substrate, coating a core layer over the bottom clad layer, patterning the core layer to form a core structure, and forming a top clad layer over the bottom clad layer and core structure. The bottom clad, core and top clad layers may be formed from compositions in liquid form which include solvent and polymer components. Where the waveguides are to be formed as part of a printed wiring board, use by the board manufacturer of a specialized coating tool such as a roller coater, curtain coater, slot coater or spin-coater is typically required. Once the liquid composition is coated, the solvent is removed from the coating by a drying process. The solvents used in the liquid compositions may be flammable and/or explosive in nature, and may additionally be deemed environmental pollutants. As a result, board manufacturers must take measures to contain or treat the solvent and maintain its vapor concentration at safe levels. Such measures include, for example, solvent collection, incineration with or without catalytic converters and use of explosion-proof equipment. The costs associated with these activities can be significant. It would thus be desired for component manufacturers to have at their disposal an optical material which is in an easily-applicable form and is solvent-free or extremely low in solvent-content.
Formation of optical waveguides on a printed wiring board using a series of pre-cast, or dry-film, layers has been proposed. For example, International Publication No. WO 03/005616 discloses methods of forming multi-level printed wiring boards that integrate optical data communications with other boards without electrical connections. Optical waveguides are formed on the printed wiring board by laminating the entire top surface of the printed wiring board with a first polymeric optical conductive layer, a second, higher refractive index polymeric layer, and a third layer of the first layer polymer material. As understood, the disclosed polymeric material is acrylate-based. There are, however, various drawbacks associated with the use of acrylates in forming optical components. For example, acrylates are generally not suitable for use in high temperature applications, for example, in chip-to-chip applications. At temperatures approaching 200° C., most acrylate materials begin to decompose and depolymerize. Moreover, acrylates are structurally and optically dissimilar to glass, which is the current material of choice for optical fibers and pigtail structures.
The aforementioned U.S. Pat. No. 6,731,857 discloses silicon-based photoimageable waveguide compositions which can be coated as a dry-film. These photoimageable compositions include silsesquioxane units of the formula (RSiO1.5), wherein R is a side chain group selected from hydroxyphenyl and hydroxybenzyl. Photoimageable compositions lend themselves to the formation of patterned waveguide core structures. Patterning of the core structures involves exposing a layer of the photoimageable composition with actinic radiation through a photomask, followed by development of the exposed layer and thermal curing. The capital cost and labor associated with the use of exposure equipment, however, can be significant. It would therefore be desirable to use a lower cost alternative and one having fewer process steps in forming a waveguide clad layer using a silsesquioxane-containing polymeric dry-film.
There is thus a need in the art for optical dry-films and for methods of forming optical components which overcome or conspicuously ameliorate one or more of the foregoing problems associated with the state of the art.